Multi-speed power transmissions are utilized with motor vehicles to extend the useful operating range of an internal combustion engine. Each of the speed ranges permits the engine to operate through approximately the entire power range thereby permitting the vehicle to be driven at output speeds from an idle condition to a maximum forward speed or to a maximum reverse speed. The gear ratios and engine power range permit overlapping of the power transmission such that the engine need not be fully extended prior to changing to the next speed up or down from the current operating position.
The power transmissions currently in use are generally either countershaft transmissions or planetary transmissions. The countershaft transmission can have the various speed ratios controlled by synchronizer type clutches or by fluid actuated type clutches. The planetary gear arrangements utilize selectively engageable friction devices, such as clutches and brakes, to establish as many speed ratios a planetary gear arrangement is capable of attaining.
There are also some combinations of planetary and transfer gear or countershaft gearing assemblies. These assemblies generally utilize a compound planetary gearset or a plurality of interconnected planetary gearsets.